Ground penetrating radar (GPR) systems, also sometimes referred to as ultra wideband radar, are generally known and used in a wide variety of applications to examine the properties of an area of ground or other medium under test. These instruments include one or more sets of transmitting and receiving antennas. The transmitting antenna transmits a pulse of electromagnetic waves into the medium under test. After propagating through the test medium where they reflect against objects (e.g., pipes) and interfaces (e.g., concrete and soil interfaces), the transmitted waves are received by the receiving antenna. Properties of the transmitted waves such as amplitude, frequency, phase and polarization change during the propagation through the test medium. The signal received at the receiving antenna is then processed by the system to generate information displays that can be reviewed to determine the nature and conditions of the test medium such as the properties of the test medium and the location of objects buried in the medium.
In its most common implementation, GPR is a time domain radar system characterized by relatively high bandwidth and high sensitivity. Typically the fractional bandwidth is in excess of about 0.8 or 1.0., and the sensitivity is greater than 80 dB. Center frequencies of the transmitted waves range from a few megahertz up to about 4-6 GHz. For example, relatively low frequency GPRs (e.g., 20-200 MHz) can be used for deep geological investigations, mid-frequency instruments (e.g., 200 MHz-1 GHz) can be used for geotechnical investigations, and relatively high frequency instruments (e.g., higher than 1 GHz) can be used for high resolution (e.g., concrete) applications.
The received signal is typically digitized before being processed. 16-bit analog-to-digital (A/D) converters are commonly used for this purpose in commercial GPRs. Although a 16 bit A/D converter can theoretically provide a sensitivity of 96 dB, GPRs with these converters typically show less sensitivity due to the wide bandwidth and bandwidth connected noise as well as to internal system noise and external noise.
Commercially available A/D converters cannot be operated fast enough to digitize the received signals at a sufficiently high rate to derive the desired information from the signals in real time. Instead, transmitter and receiver pairs are typically operated in a synchronous manner with the transmitter transmitting one pulse for each sampled data point to be collected from the associated received signal. For each transmitted pulse from the transmitter the receiver sampling point is moved slightly in time, thereby effectively sampling the received signals with a sampling period equal to the time movement of the receiver's sampling point between the samples. The wavelet or set of sample points representing a “received signal” or one waveform are therefore derived from many different received signals. This approach is known as stroboscopic, repetitive or equivalent time sampling. Since a single sample of the received signal is collected for each activation of the transmitter, it follows that the frequency of the transmitter activation, a parameter known as the repetition rate, is equal to the actual data capture rate of the receiver. GPR repetition rates can vary over a wide range (e.g., 10 kHz-1 MHz), and are often in the range of 100-400 kHz.
The time required to gather a wavelet is therefore directly related to the repetition frequency. For example, if the repetition frequency is 100 kHz and the desired number of digitized points is five hundred, the total time required to collect the sampled data points for one waveform is 5 msec. Assuming the same number of desired data points, the time to capture a waveform decreases linearly with increases in the repetition rate. Output power of the radar, as measured by certifying organizations, also increases linearly with repetition rate. The result is that GPRs can be somewhat limited in speed in certain applications. Vehicle mounted devices, for example, are often run at lower than optimal speeds due to limitations of the repetition frequency. The linear dependence of the total emitted power with the repetition frequency, together with regulatory emission level constraints, can impact the commercial availability and use of GPRs. Raising the repetition frequency above 1 MHz to compensate for slower speeds has proven to be difficult due to high power consumption in the transmitter electronics and regulatory constraints on emissions. Because of interference between them, it is also difficult to operate GPRs with more than one transmitter/receiver antenna pair or to operate more than one GPR close to each other. These proximity interference-related problems can be alleviated by synchronizing the operation of the transmitter/receiver antenna pairs into different time slots.
Known GPRs also employ an averaging methodology known as stacking. Stacking is implemented by collecting sets of data points for several wavelets and averaging the corresponding data points of the sets to calculate an average value of each data point. The set of data points representative of a given wavelet is therefore an average of the sets of data points for several corresponding wavelets. Stacking effectively reduces the noise and thereby increases the signal/noise ratio of the signals being processed for analysis, and can thereby effectively increase the depth penetration of the GPR system. Unfortunately, stacking is done at the expense of the effective data collection rate.
Changes in the triggering or firing rate of a GPR transmitter can produce disturbances in the emitted electromagnetic pulses that degrade the overall accuracy of the system. During use, even when not being operated to collect data at specific positions on the ground, GPR systems are often kept running to help maintain steady thermal conditions of the transmitter and other electronics. Operation in this manner enhances the transmitter firing rate stability of the GPR.
There remains a continuing need for improved GPRs. In particular, there is a need for GPRs with enhanced speed and sensitivity. A GPR capable of providing these advantages within regulatory emission guidelines would be especially advantageous.